1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly to a plasma display panel driving method and apparatus for improving contrast characteristics and preventing a low discharge making a cell non-luminous at a specific gray scale.
2. Description of the Background Art
A plasma display panel (PDP) displays images by radiating phosphors by ultraviolet rays generated during a discharge of a mixture gas of He+Xe, Ne+Xe, He+Xe+Ne, etc. The PDP is easy to make its thickness thin and its display screen size large, and its picture quality has greatly been improved due to a recent technical development.
Referring to FIG. 1, a conventional three-electrode AC (Alternative Current) surface-discharge type PDP includes scanning electrodes Y1 through Yn, sustaining electrodes Z, and address electrodes 1 through Xm which are perpendicular to the scanning electrodes Y1 through Yn and to the sustaining electrodes Z.
Cells 1 for respectively displaying one of red (R), green (G) and blue (B) are formed at points where the scanning electrodes Y1 through Yn, the sustaining electrodes Z and the address electrodes X1 through Xm intersect. The scanning electrodes Y1 through Yn and the sustaining electrodes Z are formed on an upper substrate (not shown). A dielectric layer and a protective layer of magnesium oxide (MgO) are formed on the upper substrate. The address electrodes X1 through Xm are formed on a lower substrate (not shown). Barrier ribs are formed on the lower substrate to prevent horizontally adjacent cells from interfering with one another optically and electrically. A florescent material layer is coated on the surfaces of the lower dielectric layer and the barrier ribs. The florescent material layer is excited by an ultraviolet ray and irradiates a visible light ray. A mixture gas of He+Xe, Ne+Xe, He+Xe+Ne etc. for a gas discharge is injected into a discharge space formed between the upper and lower substrates.
In order to achieve a gray scale of an image, the PDP is driven on a time-division basis by dividing one frame into sub-fields each having the different number of light emissions. Each sub-field is again divided into a reset interval for resetting the entire screen, an address interval for selecting a scanning line and selecting a cell in the selected scanning line, and a sustaining interval for achieving a gray scale according to the number of discharges. For example, if it is desired to display an image by 256-level gray scale, one frame interval corresponding to 1/60 seconds (16.67 ms) is divided into 8 sub-fields SF1 through SF8, as shown in FIG. 2. Each of the 8 subframes SF1 through SF8 is further divided into the reset interval, the address interval and the sustaining interval as described above. The reset and address intervals in each sub-field are identical with respect to the respective sub-fields, whereas the sustaining interval and the number of sustaining pulses assigned thereto increase at the rate of 2n (where n=0, 1, 2, 3, 4, 5, 6 and 7).
FIG. 3 illustrates an example of a driving waveform applied to the PDP.
Referring to FIG. 3, in a convention PDP driving method, cells for respective sub-fields SFn and SFn+1 are initialized by creating a set-up discharge using a ramp-up waveform and creating a set-down discharge using a ramp-down waveform.
During the reset interval of each of the sub-fields SFn and SFn+1, a ramp-up waveform is simultaneously applied to all the scanning electrodes Y, and at the same time, a 0V (zero volts) voltage is supplied to the sustaining electrodes Z and the address electrodes X. By this ramp-up waveform, a se-up discharge occurs between the scanning electrodes Y and the address electrode X and between the scanning electrodes Y and the sustaining electrodes Z within the cells of the entire screen. By this set-up discharge, positive wall charges are created on the address electrodes X and the sustaining electrodes Z and negative wall carriers are created on the scanning electrodes Y.
After the ramp-up waveform is supplied, a ramp-down waveform falling from a sustaining voltage Vs lower than a set-up voltage Vsetup of the ramp-up waveform to a negative specific voltage is simultaneously applied to the scanning electrodes Y. At the same time, a first sustaining bias voltage Vz1 is supplied to the sustaining electrodes Z and a 0V voltage is supplied to the address electrodes Z. The first sustaining bias voltage Vz1 may be defined as the sustaining voltage Vs. When the ramp-down waveform is supplied, a set-down discharge occurs between the scanning electrodes Y and the sustaining electrodes Z. This set-down discharge erases excessive wall charges unnecessary for an address discharge out of the wall charges generated during the set-up discharge.
During the address interval of each of the sub-field SFn and SFn+1, a scanning pulse Scp of a negative write voltage −Vw is sequentially applied to the scanning electrodes Y and at the same time a data pulse Dp of a positive data voltage Vd synchronized with the scanning pulse Scp is applied to the address electrodes X. The scanning pulse Scp swings between a positive bias voltage Vw lower than the sustaining voltage Vs and the negative write voltage Vw. The voltage of the scanning pulse Scp, the voltage of the data pulse Dp and a wall voltage generated during the reset interval are added to create the address discharge within the cell to which the data pulse Dp is supplied. During this address interval, a second sustaining bias voltage Vz2 lower than the first sustaining bias voltage Vz1 is supplied to the sustaining electrodes Z.
During the sustaining interval of each of the sub-fields SFn and SFn+1, a sustaining pulse Susp of the sustaining voltage Vs is alternatively applied to the scanning electrodes Y and the sustaining electrodes Z. The cell selected by the address discharge creates a sustaining discharge, that is, a display discharge between the scanning electrode Y and the sustaining electrode Z whenever each sustaining pulse Susp is applied, as the wall voltage within the cell is added to the sustaining voltage Vs.
After the sustaining discharge is completed, an erase signal for erasing the remaining charges within the cell may be supplied to the scanning electrodes Y or the sustaining electrodes Z.
In the driving waveform shown in FIG. 3, the set-down voltage of the ramp-down waveform at a time t1 when the set-down discharge is completed is fixed to a voltage higher than the negative write voltage Vw of the scanning pulse Scp by ΔV. Since the ramp-down waveform serves to reduce the positive wall charges on the address electrode X which are excessively accumulated by the set-up discharge, if the set-down voltage of the ramp-down waveform stops at a voltage higher than the negative write voltage Vw, more positive wall charges may remain on the address electrode X. The driving waveform shown in FIG. 3 can lower the voltages Vd and Vw necessary for the address discharge, and therefore, the PDP can be driven at a low voltage. The reason why the voltage supplied to the sustaining electrode Z is lowered to Vz2 during the address interval is to compensate for the amount of the positive wall charges remaining excessively on the sustaining electrode Z when the set-down voltage is raised to ΔV during the set-down discharge.
FIG. 4 illustrates another example of a driving waveform applied to the PDP.
Referring to FIG. 4, the n-th sub-field SFn initializes cells by a set-up discharge and a set-down discharge, and the (n+1)-th sub-field SFn+1 initializes the cells by the set-down discharge without the set-up discharge.
The address interval and the sustaining interval in each of the sub-fields SFn and SFn+1 are substantially the same as those shown in FIG. 3.
During the reset interval, the n-th sub-field SFn initializes cells by creating the set-up discharge using the ramp-up waveform and then creating the set-down discharge using the ramp-down waveform. Meanwhile, the (n+1)-th sub-field SFn+1 initializes the cells by supplying to the scanning electrodes Y the ramp-down waveform connected to the last sustaining pulse of the scanning electrodes Y. Unlike the n-th sub-field SFn, the (n+1)-th sub-field SFn+1 creates the set-down discharge after the sustaining discharge without the set-up discharge. Since the set-up discharge does not occur during the reset interval of the (n+1)-th sub-field SFn+1, light is emitted only from on-cells where the sustaining discharge occurs in the n-th sub-field SFn. Therefore, the driving waveform shown in FIG. 4 has higher contrast characteristics than the driving waveform of FIG. 3 in which the set-up discharge occurs in all the sub-fields and light is emitted from all the cells.
However, the driving waveform shown in FIG. 4 is liable to undergo a low-discharge phenomenon that on-cells are not driven at a specific gray scale when the amount of space charges is small in space and time because of the sub-fields having no set-up discharge. For example, in <Table 1> shown below, a cell to which data is supplied in a gray scale 4 should be an on-cell in the third sub-field SF3. However, a discharge may not occur because there are almost no space charges. Further, a cell to which data is supplied in a gray scale 8 should be an on-cell in the fourth sub-field SF4. However, a discharge may not occur because there are almost no space charges. FIG. 5 illustrates a low-discharge phenomenon appearing at a specific gray scale when the PDP is driven by the driving waveform of FIG. 4. In FIG. 5, a reference symbol W designates white chromacity.
TABLE 1GrayscaleSF1(1)SF2(2)SF3(4)SF4(8)SF5(16)400100(0)510100601100711100800010(0)910010100101011110101200110131011014011101511110
In <Table 1>, 1 and 0 designate a light emitting cell and a non-luminous cell, respectively, in each sub-field depending to the gray scale. Parenthesized numerals in the uppermost row designate a luminance weighting value assigned to each sub-field.